the interjection

The interjection, the eighth part of speech, expresses strong emotions or
feelings. Often found at the beginning of a sentence, an interjection is usually
followed by either an exclamation mark (for strong emotions) or a comma
(for mild emotions). An interjection can also be used to protest or command.
Though interjections can stand alone, they are often contained within larger
groups of words.

Wow! That was a close call. (strong emotion)
Oh, you are correct. (mild emotion)

Note: Good writers choose their interjections wisely for they know that too
many interjections can decrease the writing’s power and total effect.
Here is a list of the most common interjections.